As is known, various types of structures have doors with latches for securing the doors closed. In their simplest form, latches for doors commonly have a bar and a catch mounted on a door and door jamb. Either the bar or the catch is pivotably mounted for movement between a bar holding and bar releasing position. For example, with simple gate latches one merely lifts a pivotable bar or catch to release it from the other member. Upon swinging the gate back to its closed position, a camming leading edge of at least one member causes one member to be cammed over the other member and then fall behind and into a held position.
Latches on other structures such as pressurized oven doors, refrigerator doors and the like are of more sophisticated design for enhanced safety and reliability. For example, the camming lever latch is generally the preferred latching mechanism for doors of pressure chambers because it provides for good gasket compression and sealing of the door to maintain internal chamber conditions at a desired level of isolation from ambience. Sealing of pressurized chambers is typically provided by use of a rubber gasket which lines the perimeter of the chamber door or opening. When using a camming lever latch, engagement of a camming member mounted on the chamber door with a catch mounted on the door jamb, causes the door to be drawn toward the jamb with positive force providing firm gasket compression. This in turn provides superior sealing of the door. However, when the door of such a pressurized oven chamber is rapidly opened super heated steam can suddenly escape and burn the operator. To reduce this hazard some manufacturers have developed devices which allow for only limited opening of the chamber door in a first door opening stage while yet restraining it to allow circulating steam to be vented from the chamber in a controlled manner before the door is fully opened in a second stage. For instance, some safety doors use a separate edge-mounted mechanical device which requires a second operator action to fully release the chamber door. Other devices provide two stage opening with a single latching unit by use of a latching hook that engages a latching pin in alternate positions, or by use of a dual position roller that engages a double latch strike. None of these devices, however, provides the firm gasket compression and sealing of a camming lever type latch in a single unit.
Accordingly, there remains a need for a safety door latch for pressurized ovens that provides the sealing alacrity of a camming lever latch and yet also a two-stage opening operational feature. It is to the provision of such a latch that the present invention is primarily directed.